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I've designed and built my own AB class audio amplifier circuit. I designed it to have as low THD as possible, using LT Spice to run my simulations with the .four command, so I created a bunch of circuits and picked the one with the lowest THD.

I want to know how accurate is the THD value provided by LT Spice compared to reality? (I don't have any instrument I could use to measure THD) Has anyone compared the LT Spice reading with a real measurement of distortion using proper instruments?

This is my LT Spice set up:

enter image description here

EDIT: This is the output stage, I'm using real MOSFET and BJTs devices (with the .lib files provided by the manufacturers), behind that there is a tone control - not relevant for this matter.

It's rated 80W over an 8 ohms speaker.

enter image description here

And this is what I get:

Total Harmonic Distortion: 0.000533%(0.004577%)

This is quite low, what makes me think that could be wrong.

PS: Why is LT Spice providing two values as output, is the second one between parenthesis THD+N?

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  • \$\begingroup\$ It would help if you provided the circuit that resulted in those values. Are you using generic transistors and/or IC;s or real devices? A THD of only 5 ppm is probably not realistic (almost impossible to measure in the real world anyway) unless you are using ideal devices. \$\endgroup\$ – Barry Jul 11 '20 at 18:09
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    \$\begingroup\$ Keep in mind that if you use multiple transistors of the same part number then they are perfectly matched, which you will not achieve in reality. \$\endgroup\$ – Elliot Alderson Jul 11 '20 at 18:21
  • \$\begingroup\$ Don't forget to model the source resistance of the power supplies. \$\endgroup\$ – Elliot Alderson Jul 11 '20 at 18:31
  • \$\begingroup\$ Thanks. I have edited the question with the circuit schematic. \$\endgroup\$ – fcoppede Jul 11 '20 at 18:31
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    \$\begingroup\$ youtube.com/watch?v=4G1Ns4D_Ib0 \$\endgroup\$ – G36 Jul 11 '20 at 18:38
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Given the open loop gain of the OPA197 is over 1,000,000X you might indeed have only 5ppm distortionAT LOW FREQUENCIES. This ignores any gain of the bipolar current_input discrete power amplifier.

With UGBW of 10MHz, the opamp alone should provide 10 ppm at 1,000Hz into a light load.

For excitement, examine the transient linearity and settling of you current mirrors. Use a 1us input risetime, and then a falltime.

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